NEC CABLE AND IEC CABLE SIZING CALCULATIONS.

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The NEC specifically address Continuous Loads and requires 125 % increase in the Ampacity.
Plus additional Ampacity derating factors for Ambient temp. and the spacing / separation / cables touching .
Apparently the IEC does not consider continuous duty and requires only the circuit Ampacity be corrected for the method of installation and provides detailed tables for installation method. Example correction. tables are provided for , direct burial, depth below ground, ground temperature , soil thermal resistivity, and spacing between cables. Apparently this is how IEC derates the cable Ampacity. There is no correction for continuous load. I have seen IEC cable calculations with the derating tables applied to correct the Ampacity and then also 125 % included for continuous duty. I do not believe the IEC requires this additional 125 % correction. Any experience you can share on this would be very helpful. Thank you in advance.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Ampacity it is the current carrying capacity of the conductor at insulation rating temperature in installation standard conditions. The correction factors are employed in order to adapt the ampacity from a table to the new different conditions of installation. The 125% requirement it is for overcurrent protection setting.
NEC 210.20 Overcurrent Protection. Branch-circuit conductors
and equipment shall be protected by overcurrent protective
devices that have a rating or setting that complies with
210.20(A) through (D).
(A) Continuous and Noncontinuous Loads. Where a branch
circuit supplies continuous loads or any combination of continuous
and noncontinuous loads, the rating of the overcurrent
device shall not be less than the noncontinuous load plus
125 percent of the continuous load.

IEC 60364-5-52 Annex A(normative) Current-carrying capacities
A.52.1 Introduction
(523.1.1 Scope, in part)
The requirements of this annex are intended to provide for a satisfactory life of conductor and
insulation subjected to the thermal effects of carrying current for prolonged periods of time in
normal service. Other considerations affect the choice of cross-sectional area of conductors,
such as the requirements for protection against electric shock (IEC 60364-4-41), protection
against thermal effects (IEC 60364-4-42), overcurrent protection (IEC 60364-4-43), voltage
drop (clause 525 of this standard), and limiting temperatures for terminals of equipment to
which the conductors are connected (clause 526 of this standard).
That means the ampacity it is a cable property and it is not connected directly to protection setting but by user requirements.
 
I was asking for experienced confirmation that the IEC has no special consideration for a continuous load. We know that the NEC requires 125% plus all the other derating factors for ambient ( more than 30 C )and number of ccc ( more than 3 ) in the same raceway. Best electrical equipment in the World is made in the USA. But the NEC makes sizing mv cables much more difficult than the IEC who have published tables for Ampacity correction factors for all methods of installation.
 
I have seen IEC cable size calculations for MV cables buried in the ground, where the Ampacity correction tables were used to correctly correct the cable Ampacity. And 125 % was then applied as the load was considered CONTINUOUS. This may have been considered good engineering design. I believe this is not an IEC requirement and was asking for confirmation from experienced members. Hope this clarifies.
 
The IEC has published tables for derating the ampacity of conductors. And has no rule for 125 % increase for continuous loads.
Manufacturers of metric cables offer excellent tables for derating requirements .
Example. Ducab Cable. Co.
Typical MV Cable installation below ground are single core cables or multicore cables directly buried.
Or 3 single core cables installed in a single duct or 3 core cable installed in a single duct.
The application of the cable derating tables makes the corrected ampacity of the cable straight foreword. The NEC on the other hand makes this much more complex in my opinion.
As example can you please. offer step by step procedure to calculate the ampacity of several 3 - 1/ C. 15 KV CABLES installed in an underground trench. And include the overall ampacity derating factor, if you have experience to share. Thank you.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
It imposible to transmit the excel file. I get this anouncement:

The uploaded file does not have an allowed extension. The following extensions are allowed: .txt, .pdf, .png, .jpg, .jpeg, .jpe, .gif
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
It imposible to transmit the excel file. I get this anouncement:

The uploaded file does not have an allowed extension. The following extensions are allowed: .txt, .pdf, .png, .jpg, .jpeg, .jpe, .gif

The only work around I know is to change the ext to pdf---don't save as a pdf but change the file from .xls (or whatever it is) to.pdf and upload it and remind us in your post what ext to change it back to.

You can also save it to google drive and give the link
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
This is pdf file. Can you Jeff to use it?
 

Attachments

  • 9x3x 35 kV cables unedrground duct bank.pdf
    164.5 KB · Views: 3

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Here another attempt
 

Attachments

  • 9x3x1x35kV cables in undergrounds ducts.pdf
    432.3 KB · Views: 2

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Nevermind I see you saved it as a pdf. That works but not the same as a spreadsheet as it is a bit messy

Save the file as an excel but rename it.. so if the file was Cables in Underground.xlsx then change it to Cables Underrground.pdf. That would be change manually by clicking on the name.

We can then download it and save it back to the original extention , in this case xlsx
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Thanks. I'll try another one including formulas
 

Attachments

  • 9x3x1x35kV cables in undergrounds ducts.pdf
    597 KB · Views: 5

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
@Julius Right You did the same think. You are saving it as a pdf. That will not work. Save it as an excel file then manually change the extension that is after the dot to pdf and let us know what the original ext was...

Example

Save as Cable.xlsx or something similar which is excels extension

then highlight the xlsx and change to pdf--looks like this Cable.pdf

then upload and let us know if the file was an .xlsx

When we download it then we change the .pdf to .xlsx if that is what it was saved as and it will work

We will end up with Cable.xlsx and then it will open as an excel file on our end
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Now it is better?
 

Attachments

  • 9 ducts 3 single core cables in each duct.pdf
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